As is well known in the aeronautical field, the efficiency of the rotor blades and particularly the turbine blades of a gas turbine engine is adversely affected by the leakage of the engine's working medium between the tips of the rotor blades and the outer air seal or shroud surrounding the tips. Obviously the portion of working medium that leaks, which portion would otherwise pass through the working blades, is wasted energy resulting in the degradation in performance of that rotor stage and hence, the performance of the engine. Over the years there have been numerous attempts to reduce the size of the gap between the tips of the rotor blades and the outer shroud or outer air seal either by active clearance control or passive clearance control in order to enhance performance of the engine.
Active clearance control includes an external control mechanism (open or closed loop) that effectively reduces the gap by controlling a medium that either heats or cools the component parts of the rotor assembly to either shrink or expand the case or the rotor disk or blades so as to move the rotating and stationary components toward or away from each other. Obviously, the control must avoid the pinch point where that the parts that expand at rapid different rates interfere with each other in order to avoid rubs which may cause damage to the engine. An example of an active clearance control is disclosed and claimed in U.S. Pat. No. 4,069,662 granted to I. H. Redinger, Jr. et al on Jan. 24, 1978 entitled "Clearance Control for Gas Turbine Engine" assigned to United Technologies Corporation, the assignee common to this patent application.
Passive clearance control, which is the subject matter of the present invention, utilizes the available working or cooling medium in the engine and without any control mechanism, effectively reduces the effective gap between the tips of the blade and the outer air seal. Examples of passive clearance controls are disclosed in U.S. Pat. No. 4,390,320 granted to J. E. Eiswerth on Jun. 28, 1983 entitled "Tip Cap for a Rotor Blade and Method of Replacement" and U.S. Pat. No. 4,863,348 granted to W. P. Weinhold on Sep. 5, 1989 entitled "Blade, Especially a Rotor Blade". Each of these patents disclose means for aerodynamically reducing the effective gap by injecting cooling air discharging from internally of the blade to a location that will effectively create a buffer zone to prevent the gas path from leaking and hence, bypassing the working area of the blade.
As mentioned in the above paragraph, the present invention is concerned with passive clearance control by aerodynamically reducing the effective gap between the tips of the rotating blades and the adjacent static structure. To better appreciate the present invention one should contrast the present invention with the state-of-the-art passive clearance controls. (The patents alluded to in the above paragraph are examples of state-of-the-art designs). As noted, there is a certain clearance that exists between the tips of the rotor blades and the static structure. As will be appreciated the aerodynamics of the blade inherently sets a static pressure differential across the blade tip that allows the leakage of the mainstream gas to bypass the blade's working area and flow through the gap. This tip leakage is the largest single source of energy loss in the rotor stage and the engine. As is well known, in certain cases, the clearance is set by transient conditions or mechanical constraints and hence, the designer has to live with the clearances and accept the penalty resulting thereby.
In heretofore known designs, it has been shown that coolant air ejected from internally of the blade toward the blade's pressure side creates a buffer zone so as to decrease the effective gap even though the physical clearance remains the same. This passive clearance control, obviously, increases the efficiency of that rotor stage. While this design works well for first turbine blades (the turbine mounted Just aft of the engine's combustor) that have a plentiful supply of internal cooling air, this design is inadequate or unavailable for rotors that either lack cooling air or haven't a sufficient amount. Thus, uncooled, unshrouded blades or blades that have small amounts of cooling air for root cooling but do not require tip cooling or require tip cooling but lack sufficient air for passive clearance control do not fall in the same category.
To this end this invention contemplates incorporating curved holes or slots located adjacent the tip and pressure side of the blade that serve to provide means for aerodynamically reducing the effective gap between the tip of the blades and the adjacent surrounding part.
While this invention is particularly efficacious for blades that have no or insufficient supply of cooling air, it also has application where there is an adequate supply of cooling air for tip cooling. In this environment this invention can be utilized to enhance tip sealing. In this later application where the airfoil requires tip cooling, film holes can be added to line up with the curved slots of this invention such that film air is used to provide the tip blockage flow. The curved slots are designed to breakout at the lip of the radial film holes in the airfoil such that the flow through the curved hole is at the exit coolant temperature of the film hole flow. Also, the film holes are angled to flow across the lands of the curved slots and the curved slot flow lines up with the lands of the film hole for maximum edge cooling effectiveness. The flow out of the film hole is sized to provide sufficient film flow and sufficient curved slot flow. In the case of a heavy rub of the tip of the blade in which the curved slots might smear shut, the total cooling flow is unchanged but is now 100% on the pressure side for better pressure side film which moderates the effect of the heavy rub and smearing on blade tip to enhance durability.